Various different types of masks are used to provide fresh air or oxygen to the airways of humans. A specialised category of masks is used to provide positive pressure to the human airway. Positive pressure applied in this manner has two different goals.
In a first category, positive pressure is applied to the lungs for the purpose of stabilising the lungs, and in particular for maintaining a minimum inflation level of the small air spaces in which gas transfer occurs (the alveoli). This therapy is very useful in patients with a variety of lung diseases, where the disease process tends to lead to collapse (closure of the airway containing regions of the lung).
In a second category, the positive pressure is applied to the nasal airway with the intention of maintaining the pressure in, and the patency of, the upper airway. This form of positive airway pressure is known as nasal continuous positive airway pressure (nasal CPAP). This is now the “gold standard” treatment for the condition known as obstructive sleep apnea (OSA), and also for snoring. Obstructive sleep apnea is a condition in which the upper airway closes in sleep, and does so repeatedly. Nasal CPAP, when applied for the duration of sleep, stabilises the upper airway and allows for normal sleep and normal breathing.
Masks for applying nasal CPAP, or nasal pressure support ventilation have a requirement to be able to deliver pressure and flow and maintain pressures within the mask without permitting leaks. Leaks are undesirable as they can allow the pressure in the mask to drop below a therapeutic level. Leaks may also be an irritation particularly, if the leak causes jets of air/oxygen to be directed into the patient's eye. Leaks interrupt a patient's sleep which is undesirable as interrupted sleep is known to be of much less value than uninterrupted sleep. Leaks may also be noisy. Further, as the masks are for use during natural sleep, a high level of comfort in the fit of the mask is necessary.
Numerous different types and structures of mask have been proposed to address or alleviate the problems described above, most of which are directed to achieving a good comfortable seal.
In the past few years, in order to achieve a good seal, “bubble” type gas delivery masks have been developed. One such mask is described in Australian Patent No. 643994, dated May 16, 1991. The mask described therein has a face contacting portion which is formed from an elastomeric material and is shaped to define a large bubble or dome shaped chamber. When gas is delivered through the chamber, the chamber tends to balloon outwardly and, when fitted to a patient, the face contacting portion is caused to overlay a region of the patient's face and seal three dimensionally with the contours of the overlaid facial region. For practical reasons the mask is integrated with a rigid shell-like moulding which does not contact the patient's face. The shell is provided to enable a gas supply line to be connected to the mask to facilitate fastening of the mask to a patient's face and to minimise the risk that movement of the gas supply line will disrupt the seal between the mask and the patient's face.
When designing a mask, the mask must be able to achieve an air tight seal with the subject's face and at the same time be sufficiently comfortable to be able to be worn for hours without causing discomfort to the subject and in particular to allow the subject to sleep.
Movement of the head, and subsequent dislodgment of the mask, and breakage of the seal are major problems with prior art masks. This is a particular problem when a patient lies on their side, with the side of their head on the pillow as the rigid manifold tends to contact the pillow. The contact moves the manifold relative to the patient's face, is transmitted to, and affects the integrity of the seal. The manifold can also be pushed onto the patient's nose causing discomfort to the patient.
A further problem for all masks is that an air delivery pipe must be attached to the mask at some point. Movement of the head and the pipe leads to torsion which is transmitted through the hard shell of the manifold and can cause the sealing margins of the mask to rise up and allow a leak. The above-referenced “bubble mask” patent. (Australian patent No 634994), tries to address this by having a “universal joint” between the air delivery pipe and the rigid manifold. Australian Patent No 684412 which is a development of AU 634994, by the same inventor as the earlier Australian patent No 634994, addresses this problem by making a portion of the wall containing the gas supply port exhibit a degree of flexibility that is greater than that of adjacent regions of the mask so that movement by the connecting gas supply line will be accommodated at least in part by flexing of the wall portion. Whilst both masks produce relatively satisfactory seals they are quite bulky, relatively heavy and ungainly. They have a substantial impact or “footprint” on the patient's face. Neither fully solves the problems of forces acting on the manifold causing leaks.
In existing facial masks, because the straps must anchor onto a rigid point, they are attached to the rigid manifold; the result is that typically the strap leaves the side of the face near the cheeks, and passes through air until it reaches the lug on the manifold. This “floating” part of the strap, provides a significant weakness and adversely affects the integrity of the seal when the patient's head moves. When the subject rolls onto their side, this floating part of the strap is easily distorted, and pulls on the mask and leads to a leak.
All masks have to take account of the geometry of the patient's face, in particular the geometry of the patient's nose. Most existing masks are quite bulky and can be quite obtrusive, particularly for patients who either wear glasses or wish to read while falling asleep or who have facial hair.
Beards also adversely affect the sealing of conventional masks. Often patients who suffer from sleep apnea are obliged to shave their beards if thee wish to receive treatment via a nasal mask.
One mask which does not require a patient to shave, and allows the wearing of glasses is the Respironics® Simplicity™ nasal mask, manufactured by Respironics Inc., of 1501 Ardmore Boulevard, Pittsburg, Pa. That mask provides a bubble type seal which fits over a patients nose only extending up to the bridge of the nose and around the sides. While this reduces the “footprint” of the mask on the patient's face, the reduction in the size of the sealing bubble compared with the traditional bubble masks described above reduces the area of sealing and makes the mask much more susceptible to torsional effects caused by movement of the patient's head, pulling on the gas supply pipe etc. The seal is much less “stable” than traditional bubble masks.
U.S. Pat. No. 4,782,832 adopts a different approach to the above described masks in providing what it terms a “nasal puff”. The gas delivery mask/nasal puff of U.S. Pat. No. 4,782,832 fits only in the nose of the patient and is thus of a relatively small size. The nasal puff includes a plenum chamber from which project a pair of generally conical soft synthetic gas delivery elements for insertion into a patient's nares. Each element includes a bellows type corrugated section which allow the gas delivery elements to flex and pivot relative to each other and to the plenum to fit a variety of patients. This bellows is to allow for the prong to adjust for minor differences in angle. The nasal mask is fitted to a patient by means of a harness.
One major problem with the nasal puff shown in U.S. Pat. No. 4,782,832 is that because the gas delivery elements effectively anchor the mask in place, any torsion on the mask due to twisting or movement of the patient's head, or pulling or twisting of the air delivery pipe connected to the mask is transmitted to the delivery elements and thence to the patient's nares. The layer of skin (the nasal epithelium) inside a patient's nose is highly sensitive to contact, and particularly to rubbing contact. Consequently, the anchoring of the nasal puff of U.S. Pat. No. 4,782,832 by means of the gas delivery elements extending into the nares is a fundamental flaw in the design.
The present invention seeks to provide an improved mask which reduces the relative size, weight and bulk of the existing masks and yet provides a satisfactory seal and may be held to the face with greater stability.